Traditional four-stroke piston engines generally adopt a valve actuating mechanism of actuating the intake valve and the exhaust valve by a mechanical camshaft. The valve movement controls timing by mechanical drive between a crankshaft and a camshaft, and the camshaft and each valve. Such a mechanism has proved to be simple, effective and reliable for quite a long time, and the cost thereof is relatively low. However, parameters, such as the opening duration and phase, of such a valve mechanism are constant and cannot be regulated during operation of the engine, timing cannot be changed in different operational states, and thus the potential of the engine performance cannot be brought into full play. The timing of valve opening and closing as well as the lift need regulation in the combustion process of the advanced modern internal combustion engines, and the VVT (variable valve timing) system based on mechanical and electrohydraulic control widely used for gasoline engines is the most typical prototype. However, for diesel engines, in particular, heavy-duty diesel engines, valve regulation mechanisms applicable in practice are rarely reported due to the complexity in structure and the intensity of force exerted on the regulation devices. Since the 1980s, energy and environmental issues have become increasingly prominent, and the society's demands on issues of fuel economy and noxious emissions are becoming increasingly intense, and more attention has been drawn to the issues as how to improve engine performance and thermal efficiency, and how to reduce noxious emissions. Development of a variable valve timing mechanism has become the key technology in the development of modern diesel engines.
The timing phase and duration has a direct impact on the engine's intake and exhaust performance, and they are of crucial importance for the quality of the combustion process. Issues such as fuel economy, low speed stationarity and exhaust emission under high speed power, low speed torque, idling speed fuel consumption and partial loading of the engine should be taken into consideration to select timing phase and duration. In order to achieve a sound engine performance, the timing phase should vary with rotational speed and load. The engine requires a relatively big valve overlapping angle and a later intake valve closing angle at high speed and under a heavy load so as to achieve a relatively high power output. Conversely, it requires an earlier intake valve closing angle and a relatively small valve overlapping angle at idling speed and low speed under a light load so as to achieve a good idling speed stationarity and exhaust emission performance.
Thanks to engineers' effort to reduce noxious emissions of the engine and improve the engine's operation efficiency, exhaust gas recirculation (EGR) and post-processing techniques have been developed. However, the decrease in the engine's noxious emissions has resulted in a drop of engine efficiency, while the adoption of a high boost pressure may result in an engine maximum explosion pressure that is too high. Variable valve timing can realize Miller cycle by changing the timing of valve opening and closing, and under certain operation conditions of the engine, some of the air that has already entered the cylinder may reenter the intake manifold by delaying the closing time of the intake valve and a certain air pressure is maintained under the effect of turbocharging, by which the engine's air intake efficiency can be greatly enhanced and the suction loss of the pump will be reduced. This leads to a decrease in actual compression ratio and an expansion ratio greater than the compression ratio, which improves efficiency and reduces the maximum explosion pressure of engine operation.
Compared to fixed valve timing, variable valve timing is able to provide variable valve opening and closing timing or lift at different rotational speed and load within different engine operation ranges so as to improve the engine's air intake and exhaust performance, and better meet the demands of engine's dynamic properties, economy and exhaust emissions at high and low speed and under great and small load, which can improve the engine's overall performance. In order to further improve the engine's dynamic properties and reduce noxious emissions, the combustion process thereof should usually be rearranged, and in the process of the rearranged combustion process, the quantities of air intake and exhaust and the phase of valve opening and closing should be regulated timely to achieve optimal combustion process and realize higher efficiency and cleaner combustion. The development of high technology nowadays has taken energy conservation, efficiency enhancement and low emissions of automobile engine as an integrated task of “energy conservation-high efficiency-environmental protection” for comprehensive research and technical development. The limitations of fixed valve timing cannot meet the ever-changing requirements of the present time, thereby making variable valve timing one of the focuses for the automobile engine research.
Due to the advantages of variable valve timing, it has drawn more and more attention. Overseas research institutes have conducted a great deal of research on it, and various variable valve actuating mechanisms have been invented. While some of the systems have realized the function of variable valve parameters, only a few mechanisms can be practiced with simple structures and low cost. The majority of variable valve actuating mechanisms are still at experimental stage due to cost and reliability issues. The main operation mode of the variable valve actuating mechanisms in the existing products is changing the phase of the camshaft, where significant changes have to be made to the original engines, and it is often seen in low power gasoline engines. There are two types of air intake and exhaust camshafts for high power diesel engines, i.e. “overhead camshaft” and “side/bottom-mounted camshaft”. Since “side/bottom-mounted camshaft” makes the engine's structure simple and manufacturing cost low, it is currently widely used. However, its intake and exhaust valves are usually actuated by the same camshaft, which makes it difficult to adjust the intake and exhaust valves separately. As a result, a variable valve technology for high power diesel engines is yet to be developed.